The yield and performance of analog, mixed-signal, and custom digital circuits are both very important since they directly affect the profitability of the chips containing these circuits. The yield referred to above here is the manufacturing yield, i.e., the number of manufactured chips meeting pre-determined performance criteria.
Product managers for semiconductor chip design often base decisions for the specific design of a product based on trading off performance for an increased manufacturing yield. As an example, a design for an integrated circuit may have an op-amp designed for a 70 dB gain, but the yield of the design may only be 60%. In contrast, by decreasing the gain's specification to 60 dB, the yield may increase to 90%. When the product manager examines the tradeoff, the ultimate objective is to increase the profitability of the design. On one hand, a higher performance product may be able to command a price premium, but if the yield is not sufficient, the required price may be too high for the market to bear. On the other hand, a reduction in performance may result in a reduced ability to command a price premium, but with a higher yield, the cost per item can be reduced.
To perform tradeoff analysis, a product manager typically uses a spreadsheet populated with data relating to performance specifications and manufacturing yield of a given design. When working with more than two or three specifications, such an analysis can easily become tedious and time-consuming, in particular when loosening some specifications may not actually lead to an increase in yield. Even though the manager is not interested in these dead end cases, he still has to address them.
Manufacturing yield data for various possible performance points of a design can come from different places, including actual data obtained from analysis of manufacturing yield specific to, for example, a particular fabrication facility, after the design is manufactured. This is the data that product engineers are most concerned with. Another source of manufacturing yield data can be found in circuit simulation data. This is usually a cheaper but less accurate source of yield data. The data is obtained by estimating the yield numbers by hand calculation or, more commonly, by circuit simulation, often in combination with back-end tools, such as layout tools performing critical-area analysis.
One of the simplest ways to calculate yield data is through Monte-Carlo circuit simulations followed by computation of the ratio of feasible samples vs. the total number of samples. Other estimates exist, such as those based on density estimation from Monte Carlo simulation data, or by using regression models. A yield estimate via circuit simulation is typically used by front and back end circuit designers during circuit design. Product engineers find this information useful as well because it adds highlights possible yield-related issues earlier in the design flow. Accordingly, both designers and product engineers are required to manage tradeoff between yield and specifications.
If there are a limited number of variables in the system, a designer can modify the variables and achieve an acceptable degree of optimization. However, due to the interconnected nature of designs, as the number of variables increases, the number of choices regarding design, yield, and performance for a designer increases exponentially. This results in information overload that renders optimization very difficult and impedes the evaluation process. Ultimately, such situations tend to result in a heavy reliance upon a designer's intuition, which is not desired for large-scale modern designs.
There are many instances where the design is already fixed, such as when the chip has already been fabricated. But despite being fixed, there is still opportunity to trade off yield with performance for the chip in question. That is, there is a need to measure the performance of the manufactured chips and to determine the yield with respect to different performance specifications. Someone (the designer, the product manager, or someone else) needs to make a decision on how to balance performances with yield in order to, for example, select price points for the manufactured chips.
It is, therefore, desirable to provide a system and method that provide better visibility into possible tradeoffs between performance specifications and yield values for a given electrical circuit design while it is still in the design stage. It is also desirable to provide a system and method that provide better visibility into possible tradeoffs between performance and yield for fixed designs such as already-manufactured chips.